In constructing preforms from which optical fibers may be drawn that are suitable for use as telecommunication lightguides, vapors of materials such as SiCl.sub.4, GeCl.sub.4, BCl.sub.3 and POCl.sub.3 are entrained in an oxydizing carrier gas such as oxygen. The vapor stream is then drawn into a rotating preform tube made of, for example, quartz that is being repeatedly traversed by a torch. As the vapor stream encounters the band of heat generated by the torch, oxide particles are formed which deposit on and fuse to the interior wall of the tube. After a selected number of oxide layers have been built up upon the tube, the tube is collapsed into a solid rod, by raising the temperature of the torch, from which rod fiber may be drawn. This is generally known as the modified chemical vapor deposition or MCVD process.
In conducting the MCVD process the preform tube is mounted to a lathe with each tube end held in a rotatable chuck. In order to inhibit the vapor stream from leaking to ambience and ambient air from entering and thereby contaminating the vapor stream with water vapor, a rotary seal is employed at the juncture of stationery and rotary tube elements of the lathe through which the vapor stream is introduced into the preform tube. In addition, oxygen, a component of the vapor stream itself, is often flowed under pressure over an end portion of the preform tube near the tube inlet as an added precaution to inhibit ambient air from entering the tube.
A problem associated with the just-described apparatus has been that of heat shrinkage of the preform tube during tube collapse. As the tube is collapsed radially it also tends to shrink axially due to surface tension. The lathe chucks, which typically have had three circumferentially-spaced jaws that are brought down against a heat-resistance shroud wrapped over end portions of the tube, have often not provided sufficient gripping force to prevent such axial shrinkage. As a result, preform tubes have often contracted and become out of alignment leading to uneven heating and out-of-roundness. Once a preform has taken on a significant degree of ovality it must be scrapped as unsuitable for draw into communications-grade fiber.
There are, of course, other devices available for gripping cylindrical objects in general such as pipe fittings, collets and the like. However, since the gripping device or chuck must ordinarily be metallic in order to provide resilience, spring retention and lack of brittleness, it will have a coefficient of thermal expansion greater than that of glass. As a result, the object used for directly gripping the quartz tube will expand and move radially away from the preform tube as it and the tube are heated. Non-metallic devices, such as rubber O-rings would not be usable since the temperatures achieved would well exceed their melting point. Nor would Teflon rings or ferrules be satisfactory due to their tendency to cold flow, i.e. move and assume a new, fixed shape. Other non-metallic materials would tend to out-gas. Accordingly, a need remains for a chuck suitable for holding cylindrical objects such as optical fiber preform tubes having low coefficients of thermal expansion during periods of elevated temperature. It is the satisfaction of such need to which the present invention relates.